The Home Secretary offered concessions on Tuesday to try to save Prime Minister Gordon Brown from a parliamentary defeat over plans to extend the time limit for holding terrorism suspects without charge.

Brown is pressing on with the plan even though he could face a rebellion by some members of his Labour Party and his popularity has sunk as economic problems mount.

Brown says allowing suspects to be detained for up to 42 days without charge is "the right thing" to do to protect the security of all and the liberties of each".

Home Secretary Jacqui Smith sought to placate Labour members who could mutiny by saying tighter safeguards giving parliament an earlier say in terrorism cases would ensure the limit would be used only in exceptional circumstances.

"We have been very clear that extending the period of pre-charge detention would only ever happen if there were a very grave and exceptional terrorist threat," she told the BBC.

"We have previously said that we'd ask parliament to vote on that 30 days after the Home Secretary brings it into place. I am willing to bring that vote forward," she said.

"We have said this should only ever be a power that is in place for a limited period of time and we are willing to look at limiting that period of time even further."

Brown, who lost a safe parliamentary seat last month and was beaten into third place in local elections, has been facing the danger of a rebellion by up to 50 Labour members when parliament votes on the proposal next week.

His popularity has fallen as the global credit crunch hit. The economy grew at its slowest pace in three years in the first quarter of 2008 and inflation is forecast to rise to almost 4 percent.

But Brown does not have to call a parliamentary election until 2010, when he hopes the economy will have had time to recover.

THE TIDE HAS TURNED?

A source close to Brown said the prime minister was confident the bill would pass and he wanted to stand firm on security.

Home Office Minister Tony McNulty told BBC radio: "Things do seem to be shifting back to the government's way."

The plans have been attacked by the government's former top lawyer, Peter Goldsmith, who described them as a "very serious incursion on our fundamental freedoms".

But the government was backed on Tuesday by former counter-terrorism chief Peter Clarke. He said plots uncovered since 2005 showed "the terrorist threat is growing in scale and complexity".

Four Islamist suicide bombers killed 52 London commuters in July 2005, a plot to blow up transatlantic airliners flying from London was foiled in 2006 and last year car bomb attacks in the capital and Glasgow were thwarted.

Civil rights groups say 28 days is already much longer than in countries such as the United States, Russia and Turkey. The initial proposals had also angered Islamic groups who say the powers could alienate many of Britain's 1.7 million Muslims.